Super Street Network

Project Audi S4 Part 1: Wheels, Tires and Brakes

Notice a new project car on this month's cover? Did you think we would overlook modifying what is possibly the most "tuneable" European car right out of the box? When the Audi S4 reached U.S. soil in 2000, it was an instant winner. The relatively affordable European sports sedan, with a 2.7-liter, twin-turbo 250-bhp V6 and quattro all-wheel drive, certainly drew our attention. european car named the S4 its 2000 Grand Prix award winner (see ec 08/00), and we've been waiting to get our hands on one ever since.

For this project, we're working with owner Andy Gilliland of Austin, Texas. Andy's reason for buying this car is simple--it's easy to make it go real fast.

We'll tackle a bit of each of the performance categories in this short series, including horsepower, handling, braking and driveline components. We'll also touch up some of the aesthetics so it's even more pleasing to the eye. The goal of the project is to increase the car's overall performance with minimal compromise to its ride comfort and "streetability."

A logical first step to upgrading a vehicle's performance is figuring out what size brakes will eventually be needed. This in turn will dictate the size and offset of the wheels you'll need to fit around the calipers and rotors. Do you want more aggressive pads and stainless-steel lines? Or do you want to go ballistic and install massive rotors and multi-piston calipers? For Project S4, we said, "Heck with it--let's go ballistic!"

Ingolstadt's engineers did not equip the Audi with small front brakes. The stock front rotors measure 12.6 in. and are sandwiched by two-piston calipers. However, Stoptech High Performance Brake Systems had a bigger solution: 14-in. (355mm) two-piece floating rotors with four-piston ST40 calipers. That's correct--brake rotors that are larger than some Honda wheels. Stoptech takes great pains to choose the proper piston sizes so that the overall brake feel and balance remain unaffected, and the kit was designed to work with factory rear brakes and master cylinder.

Founded in 1999, Stoptech has been on a mission to become the leading American supplier of brake components and brake systems. The company manufactures high-performance brake rotors, calipers and brake upgrade systems for the racing and high-performance street markets, and we think they're so good that european car awarded the company one of its Select Gear awards for 2001.

Stoptech's Racing AeroRotors are cast at AB&I, a 75-year-old foundry that's QS-9000 qualified, and machined at a QS-9000-registered facility. Calipers, hats and brackets are machined at Stoptech's Torrance, Calif., location (see ec 08/01 for an analysis on Stoptech brakes).

Using such tools and technology as computer -aided design, finite element analysis, computational fluid dynamics and stereo lithography, Stoptech has been able to develop a number of brake system innovations, including improved vane designs (drawing air into the rotor and accelerating it outward) and directional hats, and to rapidly create models to test new design ideas. According to co-founder Robert Lee, Stoptech's philosophy lies behind matching or bettering stock stopping distances for street cars with the advantage of much better cooling. Although 60-to-0-mph braking distances are important; they're also a function of tire and suspension performance. Stoptech is after consistency, durability and resistance to fade. The company wants the driver to know those short stopping distances will be there time and again, without the worry of brake fade.

This monstrous brake system takes time to build up heat. Trying to bed in 14-in. rotors is like trying to heat up your morning coffee with sunlight on a winter's morn. Stoptech advises doing ten 60-to-10-mph stops in a controlled environment to bed the pads to the rotors, and then four 80-to-10-mph stops, if possible. I tried 80 to 10, 100 to 10 and even slightly faster, and the brake system felt strong all the way through--minimal fade. (Log on to www.europeancarweb.com for detailed test data.)

Although Stoptech's 13.1-in. (332mm) big brake kit for the Audi will fit the Audi S4's 17-in. stock wheels, this particular 14-in. kit requires 18-in. wheels. When it comes to wheels, we wanted to keep things light and strong. After all, decreasing rotating mass is much more beneficial than simply deducting the same weight from the vehicle itself. For this reason, we limited our hunt to one-piece forged wheels, and we found Volk Racing Wheels through Mackin Industries.

Mackin Industries is a global distribution center for high-performance products, including Volk Racing Wheels. that's been around for more than 30 years, with corporate headquarters in Santa Fe Springs, Calif. Volk Racing is the top-of-the-line brand of racing wheels offered by Rays Engineering, a Japanese high-performance wheel manufacturer. Established in 1973, Rays manufactures high-end wheels for both motorsports and street use. According to Mackin Industries, Rays Engineering uses its Volk Racing name to supply racing wheels to winning factory race teams from Honda, Nissan, Toyota and Mazda in the extremely tough racing environments of the Japanese and British Car Touring Championships, Formula 3000 and even F1.

The model we chose is the sexy LE37T, a new model derived from the extremely popular TE37. The only difference to the newer model is the color combination, which includes a polished lip. Since we wanted to maintain the S4's tire width between 225- to 235mm, we opted to go with the LE37T in the 7.5x18-in. size all the way around, with a +30mm offset. Even at this size, each Volk wheel weighs only 17.0 lb.

The strength and light weight of Volk LE37Ts are accredited to Rays' extensive forging experience. The 6061 aircraft aluminum is hot forged with an initial pressure of 5,500 tons three separate times to produce the shape of the face (12 tons/mm2)--imagine the density. Following that comes the "rotating spin forging," during which the rim is spun and pressed, bringing the rim out somewhat like a vase. A full 3,500 tons of pressure is used for this process.

When the wheels are tested by JWL (the Japanese equivalent of D.O.T. for wheels), the passing criteria requires a wheel to be able to spin one million times under preloaded weight to be considered road-worthy. According to Mackin, the LE37T can spin well over two million times before failure.

Surprisingly, the wheels had enough clearance to house the much fatter Stoptech calipers and larger 14-in. rotors without wheel spacers.

Next up was choosing a tire. We wanted rubber that would hold its own with overall wet and dry performance yet, at the same time, not get worn down to a nub after a few days of testing. The Toyo T1-S Proxes was our first choice. According to Toyo, its new silica-reinforced High-Grip Tread Compound increases the friction coefficient between the contact patch and the road surface. That, combined with its all-new unidirectional tread design with slanted main grooves, results in superior water drainage for ultimate wet handling as well--we can't wait to test these tires with Audi's quattro all-wheel-drive system in the rain. (Special thanks to Wheel Warehouse for mounting the Toyo tires.)

Toyo uses rayon in its T1-S for its excellent thermal capabilities and a continuous length of nylon cord tape around the circumference of the tire over steel belts for improved uniformity and high-speed endurance. Toyo's patented CASPAN(R) Noise Analysis Technology also helps it optimize the T1-S tread design to reduce pattern noise. This tire comes with a "Z" speed rating, UTQG treadwear rating of 280, and traction and temperature ratings of AA and A, respectively.

With these high-end upgrades, not only did we get a substantial improvement in the braking and handling departments, Project S4 lost a noticeable amount of unsprung weight. With the addition of the Stoptech brake kit in the front, and the Volk wheels and the Toyo tires all around, the S4 lost a total of 13.6 lb per corner in the front, and 7 lb per corner in the rear.

The effects of these new upgrades become immediately evident. Due to the lost rotational weight from the wheels, tires and rotors, a faster turn-in response, resulting in less effort in steering input, is a noticeable side effect. It's as if you kicked your large, burger-eating friends out of the back seat, in effect making your car feel slightly more responsive and nimble.

The brakes...well, they are not of this planet. The brake pedal feel is slightly heavier yet is superb--very reminiscent of a true sports car. However, apply the brakes in a very aggressive manner, and the front of the car squats so hard and low it could be eligible for next season's somersault tryouts As a result, during high-speed braking the rear of the car ends up feeling loose because the rear tires are losing considerable amounts of traction, causing the S4's back end to slightly sway from side to side. The power of the Stoptechs, in conjunction with the added braking grip of the new Toyos, proved to be too much for the stock suspension to handle, especially in the front.

It's obvious to us Project S4 is going to need a stiffer suspension to handle these brakes. And while we're at it, we'll go ahead and reduce some of the excessive understeer this car exhibits under hard cornering, which is very characteristic in Audi cars equipped with quattro all-wheel drive. The stock suspension pushes this car like a squealing, fat pig around turns. But, we know a suspension company that will fix that--stay tuned.